1. Field of the Invention:
This invention provides a method for maintaining a substantially constant tension on a marine seismic hydrophone streamer cable to minimize towing noise due to acceleration and jerk.
2. Discussion of the Prior Art
In marine seismic exploration, a long streamer cable that contains a plurality of hydrophones is towed behind a ship. Acoustic waves are periodically generated in the water. The acoustic waves are reflected from subterranean earth layers and return to the hydrophones. The hydrophones detect the reflected acoustic waves and convert them to electrical signals. The electrical signals are transmitted to the ship where they may be processed and recorded on a storage medium such as magnetic tape.
At sea, the towing ship is subject to periodic accelerations due to heave, pitch, roll and yaw. Those erratic accelerations are unavoidably transmitted mechanically to the cable to create noise that contaminates the desired signal. The erratic ship accelerations further impose large varying tensions on the streamer cable itself.
Various systems have been devised to isolate the streamer cable from excessive mechanical shocks. One known method involves insertion of an elastic link between the cable tow line and the streamer itself. Such elastic links may have a relaxed length of up to 300 feet and are capable of stretching 40% of their relaxed length. In another technique, the cable is payed out or reeled in yo-yo fashion in response to changes in tension. For that purpose the cable storage reel is used. However a cable reel capable of storing two or more miles of streamer cable is very large. The moment of inertia of the reel is too great to permit a prompt response to rapid tension changes.
In seismic exploration, it is essential to know the geometry of the hydrophone streamer cable in relation to the towing ship, the ship being the geographic point of reference. Known systems, whether they be an elastic link, a yo-yo type system or a combination of both, permit large variations in the ship-streamer geometry, and of course, that variation is an unknown quantity. Because of that, the actual location of the streamer cable is only imperfectly known.